Pneumatic suction conveyor means for gravimetric dosing of different components have been proposed which include a plurality of storage containers each containing one component, with the storage containers being respectively connected by one delivery line to a distributor. A separator is provided at each demand station, with the separator being connected by a delivery line to the distributor, and a suction line with a stop valve is connected to a suction fan. The separators are emptiable by a closable discharge opening, and a control is provided for a demand-dependent dosing of the components to the demand stations, with each demand station including a demand indicator for providing a control signal for adjusting the distributor to a desired component and for opening the stop valve. A setting device, upon reaching a predetermined quantity of material, releases a control signal for opening a ventilating valve in the delivery line and a set time delay is provided for closing the stop valve.
Suction conveyors of the aforementioned type are used in numerous different ways in material treatment and processing. A typical use is the processing of plastic granules in injection molding machines, extruders, etc., to shape and mold plastic parts or intermediate products. The plastic granules are conveyed from storage containers to the individual demand stations, namely, the injection molding machines, in a pneumatic manner and must be separated at the machine from the feed flow. As a result of the variety of plastics to be processed, which differ with respect to their chemical structure, physical characteristics, and/or required processing, such plastics may not be mixed with one another and, consequently, such plastic processing installations have a plurality of storage containers, each respectively receiving a specific material component. Depending upon the size of the plant, the sixty or more injection molding machines may have to be employed and supplied with material. In order to permit the continuous operation of all of the injection molding machines, a pneumatic suction conveyor system is used for supply purposes, with the conveyor system being designed in such a manner that at the demand station or each machine a specific supply of material is maintained in a ready state and is supplemented as rapidly as possible when the quantity of material drops below a specific amount. This requires a constant alternation of connections and disconnections of the individual separators with respect to the injection molding machines.
To this end, the respective storage containers are connected by one delivery line to a distributor which, in turn, is connected by a delivery line with the separator at one of the demand stations. All or a large proportion of the separators are connected via stop valves to a joint suction line with a single suction blower. By virtue of the provision of the distributor it is possible to connect each delivery line coming from one of the storage containers to one of the delivery lines leading to the separators, so that each demand station can be supplied with each of the components present.
In order to store the desired quantity of material at the demand station, an empty indicator is positioned at the demand station and responds is an inadequate quantity of material is available. As a result of the response of the empty indicator, the distributor is switched to the corresponding component and the stop valve to the suction line is opened, so that the corresponding component is conveyed into the separator and from the separator passes to the demand station. When the necessary storage quantity is reached, then a setting device or full indicator scanning the material level responds, which closes the stop valve to the suction line in a manner described, for example, in DE-AS 26 14 713 and DE-OS 21 36 328.
It is also possible to remove the separator from the feed flow by closing the inlet side of the delivery line in a manner described, for example, in U.S. Pat. No. 3,386,773, although this proposal presupposes an additional ventilation of the separator in order to be able to transfer the material to the demand station.
If the full indicator responds, that is, material volume desired at the consumer means is present, then the complete delivery line between the storage container and the separator is still full of material. As the next consumer means connected in the delivery sequence generally requires a different material, it is at least necessary to suction empty the delivery line between the distributor and the separator at the end of each delivery cycle, so that undesired mixing and material supplies to a demand station do not occur. Empty suctioning is also recommended if it is necessary to cover long delivery distance, because the material settling in the case of a brief conveying interruption may lead to clogging and starting difficulties. Such residual fractions or "tailings" still present in the lines varies in magnitude as a result of the different delivery paths between the individual storage containers and the demand stations. When the full indicator responds through opening a ventilation valve in the delivery line, the residual fraction or tailings is conveyed into the separator and is separated from the feed air. The residual fractions are added to the quantity volumetrically determined or detected by the full indicator and, consequently, at each demand station, as a function of the component conveyed, that is, for example, a distance from the storage container, different stored quantities are kept available.
In installations of the aforementioned type, it is necessary to accurately know the individual quantities removed from a demand station and processed and the total quantity removed over a specific period of time, so as to permit a completely satisfactory planning and cost analysis.
Up to now the problem of accurately determining the individual quantities removed from a demand station in plants having a large number of storage containers and consuming means has been solved by adding an average quantity representing all conceivable residual fraction quantities to the relatively precisely determinable volume reached in response to the full indicator so that conclusions can be drawn with respect to the processed material volume from each delivery cycle during which the manned station has been supplied. This method is naturally imprecise and can only lead to approximate statistical. results if each demand station removes each material component in statistically constant levels. This prerequisite is only fulfilled in few cases. Moreover, the larger the plant, the less representative this volume supplement.
When supplying a single demand station with several components, it is readily possible to supply precisely determined quantities because, in this case, the delivery line is closed or ventilated immediately upstream of the separator and the material can remain in the delivery line in the manner described, for example, in DE-OS 25 28 518. However, this method fails when there are several demand stations, which are to be supplied from the same storage containers.
It is also not possible with known plants or installations to permit gravimetric dosing which enables a more precise determination of consumption.